Thermodynamic analysis of porphyrin binding to Momordica charantia (bitter gourd) lectin

Abstract

Owing to the use of porphyrins in photodynamic therapy for the treatment of malignant tumors, and the preferential interaction of lectins with tumor cells, studies on lectinporphyrin interaction are of significant interest. In this study, the interaction of several free-base and metalloporphyrins with Momordica charantia (bitter gourd) lectin (MCL) was investigated by absorption spectroscopy. Difference absorption spectra revealed that significant changes occur in the Soret band region of the porphyrins on binding to MCL. These changes were monitored to obtain association constants (K a ) and stoichiometry of binding. The tetrameric MCL binds four porphyrin molecules, and the stoichiometry was unaffected by the presence of the specific sugar, lactose. In addition, the agglutination activity of MCL was unaffected by the presence of the porphyrins used in this study, clearly indicating that porphyrin and carbohydrate ligands bind at different sites. Both cationic and anionic porphyrins bind to the lectin with comparable affinity (K a ¼ ). The thermodynamic parameters associated with the interaction of several porphyrins, obtained from the temperature dependence of the K a values, were found to be in the range: DH°¼ )98.1 to )54.4 kJAEmol )1 and DS°¼ )243.9 to )90.8 JAEmol . These results indicate that porphyrin binding to MCL is governed by enthalpic forces and that the contribution from binding entropy is negative. Enthalpy-entropy compensation was observed in the interaction of different porphyrins with MCL, underscoring the role of water structure in the overall binding process. Analysis of CD spectra of MCL indicates that this protein contains about 13% a-helix, 36% b-sheet, 21% b-turn, and the rest unordered structures. Binding of porphyrins does not significantly alter the secondary and tertiary structures of MCL.

Thermodynamic studies on the interaction of water-soluble porphyrins with the glucose/mannose-specific lectin from garden pea (Pisum sativum)

“…6). It is pertinent to note here that CD spectroscopy shows that binding of porphyrins to MCL also does not result in significant changes in the secondary and tertiary structures of the protein (19).…”

“…Detailed thermodynamic analysis of porphyrin binding to two Cucurbitaceae seed lectins, namely TCSL and MCL indicated that while the TCSL-porphyrin interaction is primarily stabilized by entropic factors (18) binding of porphyrins to MCL is mediated by strong enthalpic forces, with negative contribution from entropy of binding (19). Single-crystal X-ray diffraction studies on the interaction of H 2 TSPP with different lectins, viz., Con A, jacalin and peanut agglutinin (PNA) have revealed distinctly different modes of interaction between the lectin and the porphyrin (4, 20 -22).…”

“…Porphyrin binding to pea lectin was investigated at different temperatures by absorption spectroscopy as described earlier (19). Absorption measurements were performed on a Shimadzu model UV3101PC UV-Vis-NIR double-beam spectrophotometer using 1.0-cm path-length cells.…”

“…1. The l max and e max values for the Soret band of these porphyrins were taken from our earlier studies and were used for estimating their concentrations (17,19). All the porphyrins used in this study obeyed Beer's law up to 5 mM concentration, suggesting that under the experimental conditions they were not aggregated.…”

Chemical Synthesis and Medicinal Applications of Glycoporphyrins

“…Another example is the interaction of water soluble freebase and metalloporphyrins (639a/b, 640a/b, 641a and 74) with Momordica charantia (bitter gourd) lectin (MCL), a tetrameric galactose-specific glycoprotein. Here, significant changes in the Soret band region of the electronic absorption spectra of the porphyrins were observed upon binding to MCL [390] . The association constants for the interaction of different porphyrins with MCL at 25 °C varied between 5×10 3 M -1 and 10 5 M -1…”